1. Field of the Invention
The present invention relates to a transparent-scattering type optical device using a transparent-scattering type optical element whose light scattering characteristics change in response to an external input and a transparent-scattering type display apparatus using the transparent-scattering type optical device.
2. Discussion of Background
There have been known liquid crystal display elements as display elements whose optical characteristics are changed by the application of voltage. Especially, as liquid crystal optical elements which have widely been practiced, there have been proposed twist nematic (TN) type liquid crystal optical elements in which a pair of polarization films are used, and such liquid crystal optical elements have been used as display elements or various kinds of devices such as watches, electronic calculators, word processors, personal computers and so on.
The TN type liquid crystal optical elements with a pair of polarization films had, however, a disadvantage that when the contrast ratio was increased, there was a great light loss, hence, a dark display was provided. Although such disadvantage does not cause a serious problem in the case of a reflection type liquid crystal optical element which is inclusively used in an outdoor environment, a transparent type liquid crystal optical element which necessitates the use of a back light causes the problem of requiring a large amount of light from a back light in order to obtain a bright display.
On the other hand, as liquid crystal optical elements of a type not using the polarization films or transparent-scattering type liquid crystal optical elements, there have been known dynamic scattering mode (DSM) liquid crystal optical elements. Recently, liquid crystal optical elements using a liquid crystal and polymer composite in which liquid crystal is dispersed and held in a cured matrix has attracted attention. In these transparent-scattering type liquid crystal optical elements, however, light does not go straight on but passes through the optical element at the time of scattering, while light goes straight on and passes through the optical element at the time of passing therethrough. Accordingly, when the transparent-scattering type liquid crystal optical element is used for a reflection type, the background at the side of an observer easily appears in a transparent portion. On the other hand, when the optical element is used for a transparent type, the background behind the optical element is seen in the transparent portion. Thus, in both cases, the visibility of the optical element decreases.
Accordingly, it is desired to realize a transparent-scattering type optical element having a bright, high contrast display.
In order to obtain such transparent-scattering type optical element having a bright, high contrast display, there has been proposed a method of arranging a black absorbing face behind the transparent-scattering type optical element or a method of arranging a louver so that light having strong directivity enters from the backside. For instance, U.S. Pat. No. 4,832,458 describes that a transparent-scattering type optical device having a high contrast can be obtained by arranging a cylindrically-shaped lens behind the transparent-scattering type optical element and by arranging a light absorbing means in the vicinity of the focal point. Further, Japanese Unexamined Patent Publication No. 81797/1975 and U.S. Pat. No. 4,726,662 describe that a prism having a light absorbing face is arranged behind the transparent-scattering type optical element.
In the transparent-scattering type optical device disclosed in U.S. Pat. No. 4,832,458, it is necessary to increase the width of the light absorbing means which is disposed in the vicinity of the focal point which is the opposite side with respect to an observer looking through the lens, in order to increase the viewing angle of the observer. This causes the amount of light incident from the back side decrease. The optical device disclosed has problems that when the viewing angle was set to be .+-.30.degree. in the vertical direction, i.e. 60.degree., for instance, loss of the amount of light from an illumination means from the back side of the optical device is large as about 57% or higher; that precisely finished lenses were required and that it was necessary to arrange precisely the light absorbing means in the vicinity of the focal point of each of the lenses.
On the other hand, in the transparent-scattering type optical device using a louver, the back side in the range of the viewing angle of the observer can be a light absorbing face for the louver. In this case, it is impossible to set the viewing angle to face the front. Namely, it is necessary to set the viewing angle in the direction slightly inclined with respect to the front face of the optical device.
In the transparent-scattering type optical device disclosed in Japanese Unexamined Patent Publication No. 81797/1975, a prism with a light absorbing film is arranged so as to cause the total reflection of light from the illumination means disposed at the back side, on the front surface of the front side glass plate, whereby the light from the illumination means at the back side does not directly reach the observer. In this optical device, although the viewing angle is broadened, a bright display can not be achieved.
U.S. Pat. No. 4,726,662 describes a transparent-scattering type optical device having a trigonal prism or a cone-like prism having a face which functions as a light absorbing face.
FIGS. 3 and 4 are respectively side views of some examples disclosed in the U.S. Patent. In FIG. 3, reference numeral 21 designates a transparent-scattering type optical element, numeral 22 designates a trigonal prism having a right-angled triangle in cross section, numeral 24 designates a side of the triangle, numeral 25 designates another side which functions as a light absorbing face and numerals 29A, 29B designate the bottom of the valley portions of prisms which are arranged in a side-by-side relation. In FIG. 4, numeral 31 designates a transparent-scattering type optical element, numeral 32 designates a quadrangle cone-like prism having a right angle at its top, numeral 34 designates a side of the prism, numeral 35 designates another side which functions as a light absorbing face and numerals 39A, 39B designate the bottom of the valley portions of prisms which are arranged in a side-by-side relation.
In the case of using the trigonal prism as shown in FIG. 3, the viewing angle of an observer is substantially 90.degree. in the lower direction, while when the prism is inverted, the viewing angle is 90.degree. in the upper direction. Accordingly, the optical device disclosed in the U.S. Patent is not suitable for use in the case where an observer looks through the optical device from the front. Further, when a colored layer has to be strictly formed only on the entire surface of the light absorbing face 25 of the prism, or when a reflection layer has to be strictly formed only on the entire surface of the side surface 24, there is a danger that the colored layer or the reflection layer may extend to the adjacent surfaces, whereby it is difficult to strictly form the colored layer or the reflection layer, and accordingly, productivity becomes poor.
When the angle .PSI. formed by a side face and the bottom surface of the prism is 45.degree., the average value of the allowable angle of incident light is about 54.degree., and when an illumination means which emits non-directional light is used, only the maximum transmittance of 30% or lower is obtainable. In addition, light entering in the absorbing surface from the outside of the prism can be reflected in order to utilize it again by forming a reflection layer on the absorbing surface. However, the efficiency is not so large.
When the quadrangle prism as shown in FIG. 4 is used, the viewing angle is substantially 90.degree. in the lower direction. In this case, however, a non-visible condition is not immediately provided in view of the upper side with respect to the horizontal plane but light enters into the dark portion, so that the display portion gradually exhibits gray tone, and finally, the display portion can not be distinguished from the background. Therefore, the transmittance-scattering optical device with the quadrangle prism as shown in FIG. 4 is not suitable for the usage where the viewing angle is required in the direction facing the front. Accordingly, a transparent-scattering type optical device having the viewing angle in the direction facing the front, capable of providing a bright display and being easily manufactured, is expected.
In the prisms as shown in FIGS. 3 and 4, a number of small prisms are generally connected in series in order to reduce the thickness of the device itself and to reduce the difference in view depending on a position for looking through the optical device. However, in the optical device as shown in FIGS. 3 and 4, much labor is required to precisely machine it so that sharp valley bottom portions 29A, 39A between the adjacent prisms can be obtained, and accordingly, the productivity becomes poor. The valley bottom portion tends to show a part of a circle in cross section as indicated by reference numerals 29B, 39B. When the shape of the valley bottom portion is circular in cross section, ideal transmittance-reflection characteristics can not be obtained at the valley bottom portion and a desired contrast ratio is not obtained. Namely, when the above-mentioned optical device is used for a display apparatus, a sufficient contrast ratio for display and viewing angle characteristics which has been initially expected can not be obtained. Accordingly, a transparent-scattering type optical device having transmittance-reflection characteristics close to designed values, especially, a desired contrast ratio and desired viewing angle characteristics is desired.